Section 2 Metal Printing - 2

2. Coating viscosity control during coating The primary condition for tinplate coating is that the coating should have appropriate coating conditions. The coating film applied on the surface of tinplate by the coating machine should have good flowability, uniform thickness and Smooth surface. When the coating speed, coating amount, and baking temperature of the coating machine are fixed, the main factor that affects the appearance quality of the tinplate coating is the coating viscosity of the coating.
(1) Effect of coating viscosity on coating film When the same coating material is used in the same amount of coating machine, coating speed, and coating roller used, if the coating viscosity is not the same, the coating thickness of the coating film is not the same. . Only the appropriate coating thickness and proper baking temperature and time can make the coating achieve the desired effect.
The thickness of the coating directly affects the quality of the coating and the cost of the coating. The thickness of the coating can cause the following problems:
1 poor hiding power;
2 uneven coating film, poor corrosion resistance, poor protection;
3 coating film excessive hardening, processing and forming easily broken;
4 Reduced gloss.
If the film thickness is too thick, the following problems will also occur:
1 The same amount of paint coating area is reduced, increasing the cost, not economical;
2 The hardening is not complete, the iron sheet is easy to bond and difficult to separate when stacked;
3 Reduce the impact resistance during processing, easy to peel off when in full bloom.
Therefore, in the tinplate coating process, to obtain a good coating effect, the thickness of the coating film is negligible. However, how to control the thickness of the coating film within the desired range is a key problem to be solved in the coating operation. The key to the problem is how to control the coating viscosity in the coating operation.
Under normal circumstances, when the ambient temperature is the same, the higher the viscosity, the less volatile components in the coating, and the higher the solid content. The coating thickness of the same coating material after coating is correspondingly thicker; on the contrary, the lower the viscosity, the lower the The volatile components are large, the solid content is low, and the coating thickness after the coating of the same supply amount is relatively thin.
When the coating viscosity is too high, the coated surface of the tinplate is prone to uneven coating (such as zebra stripes and orange peels). When the coating is running at high speed, the coating is easy to fly (similar to printing ink). When the surface of the tinplate is oily, it is easy to appear. Eye hole (pinhole) and other coating problems. When the viscosity of the coating is too low, tinplate markings are likely to appear on the coated surface of the tinplate. If horizontally blanking rollers are used, the coating of the roller tends to accumulate, causing the tinplate to re-skin at this position. When the conveyer is electrostatically charged, it easily appears. Pinholes and other coating problems.
Different coatings have their ideal coating operating viscosity, only the viscosity is appropriate, when the coating is applied on the tinplate, the coating film has good flowability, uniformity, smoothness and processability.
(2) Determination of Paint Viscosity During the cycle of storage-coating-paint storage, the solvent is particularly easily evaporated in the air exposed to the solvent components, thereby increasing the viscosity. This tendency is especially noticeable for low-boiling solvents. This requires dilution of the coating, and the viscosity changes with the amount of diluent added when diluted. The more diluent added, the lower the viscosity becomes. Moreover, not only the viscosity is reduced, but also the nonvolatile content of the coating is reduced. Even if you supply the same amount of paint, the amount of coating will be insufficient. Therefore, when adding a diluent to a coating, it is very important to determine the viscosity of the coating.
The viscosity is generally measured using a No. 4 Ford Cup Viscometer. First fill the cup with paint, use a thermometer to measure the temperature of the paint at this time (generally measured when the ambient temperature can be identified as the liquid temperature of the paint), open the valve at the bottom of the cup, press the stopwatch, let the paint from the bottom The time for the hole to flow out and the end of the flow (the stopwatch is pressed when the thread-like outflow paint first breaks) is expressed in seconds. The following format is recommended when recording the viscosity of the paint:
××coating ××× seconds/number 4/××°C
In addition, the viscosity of the coating is not only related to the amount of the diluent but also to the change in the liquid temperature. The liquid temperature becomes higher and the viscosity decreases; the liquid temperature becomes lower and the viscosity increases. Therefore, while measuring the viscosity of the coating, it is necessary to measure the temperature of the coating liquid at this time. That is to adjust the viscosity must consider the addition of diluent and liquid temperature in two aspects.
The paint viscosity that is generally provided by the paint manufacturer is the value of the viscosity measured when the paint is controlled at 20°C using a No. 4 Ford cup. However, in actual operation, due to the difference in ambient temperature, the production equipment is different, especially the hardness of the coated roller is not the same. The coating viscosity in actual production is often very different from the viscosity provided by the manufacturer. This requires that before using a new coating, we must test-coat the coating, adjust the coating viscosity by adding thinners, and find out that a certain coating thickness is satisfied and the coating has good coverage. The actual production viscosity of the leveling and glossiness.
For coatings with high frequency of use, it is advisable to record the optimum viscosity in production in the format of “××Paint×××sec/4/××°C” in a routine and systematic manner. For example, the room temperature is the lowest at 4 o'clock in the morning and the highest temperature is at noon. These two time points are absolutely different when the same thickness of thinner is used to produce the same thickness coating film. The change in room temperature during the year is even greater. If there is an optimum viscosity record at different temperatures, a “viscosity-temperature curve” (see Figure 3-7) is drawn so that when the room temperature changes, the optimum viscosity at that time can be found quickly. The viscosity of the paint is adjusted to greatly increase production efficiency.
(3) The paint agitation before production is generally a colloidal solution made of a mixture of resin, solvent, coloring material, and additives. When stored for a period of time, there will be a phenomenon of delamination. Generally, the colorant and resin will be deposited in the lower layer, and the solvent and additives will float in the upper layer, especially in the colored coating. If the paint is immediately taken out for construction, the color, thickness and performance of the coating will be different.
Only by thoroughly stirring the paint, the pigments, resins, solvents, and additives in the entire barrel can be uniformly mixed, so that the paint can maintain a good and stable performance in production. Therefore, the paint must be fully stirred before use, it is recommended that the mixing time is not less than half an hour.
(4) Viscosity control in production Usually, the agitated paint can be coated without adding a diluent, but the viscosity of the paint slightly increases during storage of the paint due to changes in the room temperature. At this time, generally according to the paint manufacturer's recommendations, add a certain amount of solvent dilution, the mixing process should be fully stirred, so that the paint reaches a uniform, after stirring for a certain period of time to allow all the bubbles in the paint to escape before coating.
The correct viscosity control is: first determine the liquid temperature of the paint at this time, and then find the viscosity of the paint at the liquid temperature according to the viscosity and temperature curve table of the paint. If it is detected that the viscosity of a certain paint is higher than the viscosity value of this temperature point for 10 seconds, the correct thinner should be selected to dilute the paint. The amount of diluent used is just such that the viscosity of the paint is within the range of the viscosity of the temperature point ± 10s. .
If paint manufacturer's coating has low viscosity, when it is applied on the machine, it will be found that even if the coating amount is increased to reach the required coating film thickness, the appearance quality and other physical and chemical indicators cannot meet the requirements of the product. Process requirements. There are two solutions for this: one is to change to other coatings, and the other is to leave the coating exposed to air for a period of time to volatilize a portion of the volatile solvents in the coating, thereby increasing the viscosity of the coating.
Theoretically speaking, the control of the paint viscosity used in the field can be divided into two processes: one is to check and adjust the viscosity of the paint before the coating operation; the other is to check the viscosity once per hour during the coating process, and add thinner to make up Volatile solvent during operation to adjust the viscosity of the coating.
Theoretically speaking, the control of the paint viscosity used in the field can be divided into two processes: one is to check and adjust the viscosity of the paint before the coating operation; the other is to check the viscosity once per hour during the coating process, and add thinner to make up Volatile solvent during operation to adjust the viscosity of the coating.
In actual production, in order to reduce the labor intensity of the operator, the same effect can be obtained in the following manner: The first step is to adjust the viscosity of the paint according to the optimum construction viscosity of a certain paint at the prevailing ambient temperature before the coating operation. Record the amount of diluent added; the second step is to let the paint from the paint bucket slowly flow into the paint container device next to the coater so that the amount of outflow is just equal to the consumption of the paint film. It is estimated to be used for the first time. The time required to finish coating a container device. At a regular time point, add a little more diluent than the amount before coating to the paint container device and mix it evenly so that the added amount just meets the viscosity of the paint at the optimal viscosity, and note the amount of thinner added at this time. At the same time, it is checked whether the thickness of the tinplate coating and the relevant physical and chemical indicators of the coating at this time point meet the technological requirements of the product. If the process requirements are met, the same amount of diluent will be added in the same amount of time in the future. If the coated index of the tinplate is low and the thickness of the coating film is too thick, the time interval for adding the thinner may be shortened.
(5) Correctly selecting the diluent To adjust the viscosity of the paint, it is important to select the diluent. The wrong choice can not only cause the separation and precipitation of the coating, but also can make the coating film line, the poor flowability, and the degree of hardening is not enough, and the defects on these coating films will only be displayed after coating and baking. Reduce the quality of products invisible and cause unnecessary economic losses.
When selecting a thinner, it is best to use a mixed solvent or a designated organic solvent as a diluent for the paint factory to avoid malodor, turbidity, or sedimentation. Sometimes when the thinner of a paint is used up, the thinner of the same type of paint can be selected. Before use, take the selected diluent and paint to do the dissolution test. After fully stirring, let it stand for 15 minutes and check whether the coating solution is uniform or not. If the coating solution is free from delamination, turbidity, precipitation, etc., the new diluent can be used instead of the original diluent.
In general, alkyd resins and ester gum resins can be used as a diluent for 200# solvent gasoline; amino resins generally use a mixed solvent of xylene and butanol as a diluent; acrylic resins generally use xylene and ether. Mixed solvent as diluent; Epoxy resin, polyester, vinyl can be used banana water or strong organic solvent as a diluent. The relationship between the perceived coating and the solubility of common organic solvents is shown in Table 3-7.
Table 3-7 Solubility of Common Coatings and Common Organic Solvents